Development of Co-Ni superalloys

• Main Author: Knop, Matthias
• Other Authors: Dye, David ; Gourlay, Christopher
• Published: Imperial College London 2015
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.733053

The development and characterisation of alloys intended for use in discs in the hot section of gas turbine engines is presented. Sato et al. discovered a new strengthening phase in Co– Al–W-base alloys, Co3(Al,W), similar to Ni3Al in Ni-base alloys. This could promise a new class of alloys. In several publications, this alloy class has been investigated for its viability as a replacement for Ni-base alloys. In contrast to the literature, the alloys obtained here are aimed at polycrystal applications for discs, with good oxidation resistance and lower density. This lead to an increase in the Ni content, in order to broaden the γ′ phase region and so enable tolerance for Cr, and in order to allow substitution of (W,Ta) for Al. In the first major results chapter, a series of alloys are examined and characterised in terms of the microstructures obtained, their γ′ solvus temperature, density, flow stress and isothermal oxidation resistance. After initial characterisation, four candidate alloys were chosen and larger ingots were produced using the powder metallurgy route, with added C, B and Zr grain boundary strengtheners. The microstructures were found to be stable for 10,000 h at 800 °C, with a density of 8.4 to 8.7 g/cm3, lower than published Co-base alloys. The γ′ solvus (990 to 1010°C) is adequate but an increase would be beneficial. Compression tests from room temperature up to 900°C showed that the alloys have a good strength over a large temperature range. However, the microstructure has to be optimised. The oxidation behaviour is similar or better than Ni-base alloys currently used and significantly improved when compared to other γ′ strengthened Co-base alloys. In fatigue experiments at 650°C and 99% or 115% of the yield stress, the tested alloys have performed better than the single crystal Ni-base superalloy CMSX-4. Atom probe experiments have shown that the matrix mostly consists of Co and Cr, whereas the γ′ precipitates contain high fractions of Ni, Al, W and Ta. These results can be utilised when designing further alloys, incorporating solid solution strength- eners to improve the tensile creep life of the presented alloys, especially at higher temperatures.